The operation of motor vehicles using fuel that contains ethanol (alcohol) is already tried and tested, and, primarily in South America and North America, many vehicles are equipped for this type of operation. The concentration of the ethanol in the tank fuel can change from one tank filling to the next tank filling, depending on the fuel type that is added. Arbitrary ethanol proportions of the fuel between 0% and 100% can be produced in this context. In order to achieve perfect running of the internal combustion engine, it is necessary for the operating control device to detect the new fuel composition as soon as possible and take this into consideration as part of its control strategies.
The prior art (see e.g. U.S. Pat. No. 6,257,174 B1) already discloses the use of the different air requirement of ethanol as opposed to other fuels (ethanol: 8.9; gasoline: 14.7) for determining the ethanol proportion of the fuel. If the ethanol proportion of the fuel changes, the air ratio λ that is measured by the λ sensor changes, and the ethanol proportion of the fuel can be deduced from this change in the air ratio (λ method).
However, the λ method only allows the ethanol proportion to be reliably determined in the case of a fuel system that is intact. Since the fuel system diagnosis (FSD) is also carried out using the λ values that are measured by the λ sensor, the λ method does not provide reliable values. Specifically, the operating control device cannot definitively establish whether a deviation of the λ value is caused by a changed ethanol proportion of the fuel or an error in the fuel system.
U.S. Pat. No. 6,298,838 B1 discloses a method for controlling the irregular running of the engine, in which method the ethanol content of the fuel supplied to the engine is varied. Provision is made to measure an irregular running of the engine, and for the fuel supply to the engine to be increased by a first ethanol content if a first threshold value is exceeded. The irregular running is then measured again. If the irregular running has been reduced below the first threshold value as a result of this ethanol increase, but remains above a second, lower threshold value, the fuel supplied to the engine is further increased by a second ethanol proportion, in order to bring the irregular running below the second threshold value as well. However, if no reduction in the irregular running below the first threshold value is established as a result of increasing the fuel by the first ethanol proportion, it is deduced that the irregular running is not caused by a decrease in the ethanol proportion, but has another reason, for example a defective spark plug.
The publications DE 600 11 393 T2 and U.S. Pat. No. 4,059,411 A propose fuel compositions whose alcohol content is increased in order to extend the lean-burn running limit of the engine.